Laboratori Nazionali del Sud

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Theoretical physics

The research activity of the Theory Group at LNS belongs to the vast field of Nuclear Physics aiming at the understanding of nuclei and nuclear matter, from its normal conditions to the extreme situations of density and temperature that are reached in supernovae explosions, compact stellar objects, Early Universe evolution and in heavy-ion collisions at ions accelerators and colliders. Nuclei are a very complex many-body system with three of the four fundamental interactions at work, challenging experimentalists and theorists for many decades. Hence a sensible theory should be able to describe the nuclear spectra, as well as the electromagnetic, weak, and strong transitions that lead to the excitations of those states in nuclear reactions, or characterize their decay properties. Moreover, the behavior of nuclear matter in various conditions of density, temperature and asymmetry in the neutron-proton content, as they can be encountered in compact stellar objects, is extremely fascinating to understand. The exploration of the full nuclear matter phase diagram, and the characterization of the nuclear Equation of State (EoS) are among the mail goals of nuclear reaction studies, involving in particular heavy ion experiments.
The theoretical activity at LNS develops within three national projects of INFN (see below description and link to the pertinent web pages): STRENGTH (Structure and REaction of Nuclei: toward a Global THeory), SIM (Strongly Interacting Matter: matter under extreme conditions), NUMAT (NUclear MATter and Neutron star structure).

From the theoretical point of view, one can gain this comprehensive understanding by using effective theories. The latter are theories in which, when dealing with a very complex system, one eliminates a number of degrees of freedom from explicit consideration by introducing effective interactions, which are usually functionals of the nuclear density and momenta. These models, namely transport theories employing effective interactions, are well suited to provide a unified description of small-amplitude vibrations, like the giant resonances, and of heavy ion collisions. A vivid activity with the STRENGTH project is on-going in collaboration with several Italian and international research groups.

The determination of the symmetry energy of the nuclear EoS, i.e. of the behavior of charge asymmetric nuclear matter, can be identified as one of the most outstanding problems in nuclear physics. Such determination entails profound consequences for the study of neutron distributions in stable and exotic nuclei and impacts on diverse areas such as heavy-ion reactions, nuclear astrophysics and tests of the Standard Model via atomic parity violation. Our theoretical group is engaged in the simulation of low-medium energy reactions, ranging from fusion, collective mode excitation, deep-inelastic, transfer reactions up to multifragmentation collisions, involving in particular neutron-rich systems. This comprehensive investigation of giant resonances, as well as large-amplitude collective dynamic, is of great importance to reach a unified description of nuclear properties and in particular a proper determination of the symmetry energy behaviour. The group is involved in the study of its impact on the structure of the neutron stars within the national project NUMAT.

In particle accelerators it is possible to collide heavy-ions at such energies that a new state of matter is generated: the Quark-Gluon Plasma. The aim is to study the properties of such matter that has permeated the first microseconds after the Big-Bang and to understand how it undergoes a phase transition becoming the ordinary matter that constitutes the nucleus of our atomic matter. The Theory group at LNS has extended the theoretical techniques of nuclear physics to such ultra-relativistic regime. In particular a kinetic transport equation is able to describe the evolution of the matter created in the experiments at LHC-CERN in Geneva and at the RHIC-BNL in Long-Island, interpreting the results and suggesting new measurements . The group participates to the national project SIM devoted to such studies in collaboration with several international Institutes.

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